There's this school of thought that writers are those who write, whether published or not. Authors are those who have signed contracts with publishers to produce a book.
Within the category of writers, you have freelancers and staffers. In fact, there are so many kinds of writers and authors that we can get lost in definition. To me, the qualifications for writers and/or authors are not so strict. If you have a book, self-published or traditionally published, you are an author. If you write regularly, whether it be blogging, articles, research pieces, etc, then you are a writer.